The invention concerns paper with improved dry strength and limited resistance in a moist state (optimised) as well as its production method characterised by the joint use of at least two different polymers, one with cationic charges and the other being anionic overall. These polymers work together to exercise a synergistic action on the dry strength of this paper.
More precisely, the invention concerns an improved method for producing paper and/or cardboard and the like, according to which are used at least two (co)polymers to improve the dry strength characteristics during production of cellulose sheets and is characterised in that:                at least one of the agents is obtained by Hofmann degradation reaction on an acrylamide (co)polymer,        and at least one second agent is a (co)polymer with an anionic charge density greater than 0.1 meq/g.        
According to the invention, the product of the Hofmann degradation is an organic polymer characterised in that:                it is produced in a solution, at a concentration greater than 3.5% by weight, preferably greater than 4.5% and advantageously greater than 10%, from an acrylamide (co)polymer with a molecular weight greater than 5,000;        it has residual anionicity less than 10 moles percent and preferably less than 5 moles percent;        it is obtained by the reaction of an alkaline earth hydroxide and an alkaline earth hypohalide with a hydroxide/hypohalide mole ratio between 2 and 6, preferably between 2 and 5.        
This 2-component system can be successfully used in the production of paper and cardboard for packaging, coating base paper, all types of paper, cardboard and the like requiring improved dry strength.
There is a demand for increasingly strong paper and cardboard, notably for the packaging industry.
The dry strength of paper is by definition the strength of a normally dry sheet. The bursting strength and tensile strength values traditionally express the dry strength of the paper.
It is well known that water-soluble cationic polymers can be used to improve the strength characteristics of paper. By their very nature, they can bind directly to the anionic cellulose, giving it a cationic charge so that, combined with anionic polymers, the latter bind to the cellulose fibres, thus improving the dry strength of the sheet.
The most commonly used cationic polymers are compounds such as cationic starch, polyamide epichlorhydrin (PAE), polyamidoamine epichlorhydrin (PAAE) or cationic polyacrylamides, possibly glyoxalated.
The methods described in the prior art using these polymers, however, are not completely satisfactory, notably concerning the quantities of polymers required and/or the wet strength characteristics produced, leading to process difficulties such as grinding the sheet into a pulp for recycling dry broke. This inconvenience is notably observed when implementing the method described in document US 2004/118540.
The bursting strength of the cellulose sheets obtained using dry strength agents should also meet a certain number of requirements. It should notably have no toxicological disadvantages and should provide good compatibility with the other agents involved in producing the sheet.
It has already been proposed, notably in patent applications JP 58-60094 (Hamano) and JP 04-57992 (Mitsui), to combine the Hofmann degradation products with an anionic polymer, said combination being aimed at proposing a high-performance system for the dry strength of the paper sheet.
However, the instability inherent to the polymers obtained with the Hofmann degradation reaction on an acrylamide (co)polymer and their moderate effectiveness when seeking to improve the mechanical properties of the sheet, dissuade those skilled in the art from using the products described in these two Japanese documents.
Document EP-A-377 313 proposes combining an anionic polymer and a Hofmann degradation product produced at very low concentrations, in practice approximately 1% (see Paper making—Example 1). In practice, the degradation product is obtained at high temperatures requiring it to be produced while the sheet is being produced.
With this apparently prohibitive understanding, only very heavy methods used on-site at the paper mill, with a Hofmann degradation product unit or methods using another form of synthesis (using a (co)polymer base such as N-vinylformamide followed by hydrolysis), itself relatively costly, have been set up.